1. Field of the Invention
The present invention is directed to a method and apparatus for the recognition of pneumothorax with the assistance of thorax MR exposures.
2. Description of the Prior Art
Pneumothorax is an illness wherein the outer membrane (visceral pleura) of the lung exhibits leakage. As a result, a dead volume arises in the pleura; space or cavity between the outer lung membrane and the rib cage.
This condition can usually be identified with a thorax X-ray exposure. For this purpose, the patient must exhale to the greatest possible extent, and the radiologist can the recognize the outer lung membrane in shadowy fashion in the image, as well as the blood vessels that proceed from the middle of the lung toward the edge and end at the lung membrane. Such X-ray examinations, however, represent a general radiation stress and should currently be avoided, if possible, or greatly limited.
Due to this radiation stress, an automated method for recognizing pneumothorax that is disclosed in U.S. Pat. No. 5,668,888, and wherein X-ray exposures are likewise utilized, is only conditionally suited for practice. Moreover, this method requires extensive filter stages for blanking out the bone structures. This blanking, of course, never can ensue free of disturbance, and even the minutest disturbances in the region of the pneumothoraxxe2x80x94which already exhibits only slight attenuation differences xe2x80x94can lead to a falsification or non-recognition of a pneumothorax. Moreover, a pneumothorax lying behind a bone structure could not be recognized even after the bone structure is filtered out since, of course, it was not presented in the exposure at all.
The employment of magnetic resonance exposures for thorax examination is known from various publications, such MR exposures supplying excellent images of the lung without radiation stress. Standard MR exposures of the lung have usually been produced with the lung inflated. It has also been proposed in the case of such magnetic resonance exposuresxe2x80x94see Lelieveldt, B. P. F. et al., xe2x80x9cAnatomical Model Matching with Fuzzy Implicit Surfaces for Segmentation of Thoracic Volume Scansxe2x80x9d in IEEE Transactions on Medical Imaging, vol. 18, no. 3, March 1999, pages 218-230xe2x80x94to utilize a system wherein the boundaries between the organs and the air surrounding them can be recognized and displayed in automated fashion by means of a clear segmentation. Of principal concern, obviously, is the exact demarcation of the individual tissue structures rather than a recognition of a pneumothorax, since the tissue structures never or hardly ever yield the exact noise signal of air.
An object of the present invention is to provide an apparatus for the automatic recognition of pneumothorax using thorax MR exposures that allows a standardized, automatic recognition of pneumothorax without a complicated interpretation on the part of the radiologist.
This object is inventively achieved in a method and an evaluation device for the thorax MR exposures for segmentation of the image of the rib cage, including its surroundings, and for characterization of a zone detected between the rib cage and the lung, preferably in the exhaled condition, that has an MR signal elevated by a prescribed safety margin compared to the noise signal in air.
It is of considerable significance for the inventive method and apparatus that X-ray exposures conventionally employed for the recognition of pneumothorax are replaced by MR exposures, which represent less of a stress on the body of the patient from the very outset. The important factor, however, is that these MR exposures inherently can be obtained relative to the slice to be examined so that the (usually rather thick) slice under observation in fact covers the lung tissue but not the ribs and other bone structures that lie in front of it or behind it. As a result, superimpositions that render a pneumothorax invisible and disturbances when filtering out the bone structures that lead to the same negative result can be avoided.
The rib cage and the lung tissue, particularly the outer lung membrane as well, can be very clearly distinguished from their surroundings due to their high water and fat content, since strong MR signals occur at these locations. When a zone having a signal level comparable to the noise level prevailing outside the body occurs between the rib cage and the lung tissue, then this is a good indication of a pneumothorax.
The inventive evaluation and detection device can have a window discriminator that examines the signals of juxtaposed points in quadratic segments, averages the signals and then utilizes the zone with the lowest average value as the basis for the noise level, and which utilizes zones with significantly higher values, i.e. at least twice the noise, for recognizing the physical structure of the edge of the lung, and which chromatically marks zones with an average signal value xe2x89xa6approximately 150% of the noise level. When interconnected regions of such chromatically marked points lie in the inside of the body, i.e. specifically between the rib cage and the lung tissue, then this represents a pneumothorax. This also constitutes a critical difference over the initially described method and system of Lelieveldt wherein the segmentation ensues exactly on the signal noise of air for the purpose of delimiting the organs from the surrounding air space.